Reprinted by permission of Datsunzgarage.com

 
THE P90 HEAD

P90 waiting for the machine shop

Nissan made only three 280ZX heads from 1981-83, the P79, P90, and P90a.
The junkyards are filled with non-turbo P79 heads, while P90s and P90as are scarce and came on 1981-83 turbo ZXs. While the P90 and P90a have the same combustion chambers as the workhorse P79, they came with square exhaust ports like the Z heads from the 1970s.

 Ahhh, the 25 year argument: which is better, square or round exhaust ports? And which is better for L28 performance, the N42, N47, P79, or P90? There are a thousand opinions, so I'll try to stick to numbers. Unlike the world of high-end audio which is really subjective, head mods and their effects are easily measured and make comparisons easier.

A computer parts network could locate a P90 for you, but they would probably want $200-250 for it. If you're lucky enough to have a pick-and-pull salvage yard, try visiting it every weekend. After searching at mine for 2 years, I only saw 2 turbo cars come through.
But finally this last year, I stumbled across a '82 turbo 2+2 automatic at closing which still had it's head and oil pump!!! I couldn't sleep that night worrying that some one else might get it, so I took the next morning off work to pull off the head. Got it for $49! The oil pump was $5.95.

PERFORMANCE COMMENTS
Keep in mind that the all L28 heads (N42/N47/P79/P90) use the same valve-head diameters. The P90a is identical to the P90, but the P90a can't be used with a high-lift cam because of it's hydraulic lifters, and would cost a small fortune to rebuild (if you could find the parts). I haven't seen any flow numbers, but Nissan must have chosen the square exhaust ports for it's turbo head for a reason.
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THE P90 PROFILE

  • square exhaust ports with no liners (like the N42 and earlier heads)
  • .100" deeper chambers than N42/47
  • nearly straight exhaust runners
  • high-quench/high-swirl chambers
  • steel valve-seats

    INTAKES
    The P79 and P90/P90a combustion chambers are deeper than other L28 heads, and now I see a benefit. By making the chamber deeper, it lines up the valve bowls with the intake ports, and greatly straightens out the short-side radius. Earlier heads have a sharper "bend" as the runner transitions to the bottom of the intake valve. So the P79P90/P90a intake runners appear to flow better.

    EXHAUSTS
    While I'd never really compared round/square ports before, a square exhaust looks far superior to a round/linered one. The port floor is wide and flat and flows easily from valve to port. With the liner the flow is smooth, but the area in the port is smaller and takes a sharper bend. On the P90, you can actually put your finger straight down the exhaust runner into the valve port. The N42/47 has a short-side radius bump there which a purist would say inhibits flow.

FUEL PUMP HOLE
Strangely, while the P79 has the mechanical fuel pump hole sealed over, the P90 actually still has it (!) The ZX power steering pump flange covers it. I'm guessing the same casting dies for the square port N42 were used by Nissan. Why else would they leave the hole there?

VALVE SEATS
I'm probably wrong on this, but my P90s intake-valve seats look different. They are a highly polished chrome/steel, have very precisely shaped 3-angle grinds, and seem to extend deeper into the bowl. I don't remember my last 2 P79 heads looking this nice, I wonder if Nissan used better seats on the turbo heads?

  MY NEW MOD FOR 2001

My P90 mod followed the same mod-path as the one I used on my old P79:

  • Mild porting, contouring of valve-bowls and runners
  • shaving .080" to raise the compression to 10:1
  • 5-way valve job intakes, 3-way exhaust
  • 1978 valves, swirl polished
  • Crane stage 2 cam, 272/282 duration, 450 lift

I ended up doing a lot of engine tuning and tweaking afterwards because I didn't anticipate that the hotter cam and porting would change the fuel needs of the engine so much: the engine would run out of power at high revs.

I had been using a similar P79 with '70 N27 needles with no problems. But 240 needles are calibrated for a 1970 2.4 liter motor...a free-breathing 2.8 liter with a cam and usable redline of 7k apparently needs quite a bit more gas for high-performance.
The engine was strong, but flat and uninspiring.

Then I tried SM carb needles, and ran the idle distributor timing to about 13btdc...

The car runs like it's at Lime Rock now, with serious power coming in at 4k and instant response all the way to 7k. It bellows like a IMSA car and runs fine on pump gas. This ain't a Datsun anymore...

 
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